The present invention relates to a charge/discharge control circuit, and more particularly, to a charge/discharge control circuit of a secondary battery used in a portable electronic device.
Enhanced performance of contemporary portable electronic devices (e.g., personal computers) has created a demand for batteries having prolonged lifetime. Lithium ion batteries, which are commonly used as secondary batteries, are widely used in recent portable electronic devices. To prolong the lifetime of a lithium ion battery, charging and discharging of the battery has to be controlled. During discharge control, discharging is prohibited when the battery is overdischarged. During charge control, charging is prohibited when the battery is overcharged.
FIG. 1 is a schematic circuit diagram of a prior art charge/discharge control circuit 50. The charge/discharge control circuit 50 includes a control unit 3 and two control switches that are externally connected to the control unit 3. The two control switches are a discharge control switch 4 and a charge control switch 5a. The charge/discharge control circuit 50 controls the charge/discharge control current of a battery 1. The battery 1 is a lithium ion battery that can be used as a secondary battery, and includes three series-connected cells 2a, 2b, 2c. The battery 1 provides power to a portable electronic device, for instance.
The discharge control switch 4 and the charge control switch 5a each include a p-channel MOS transistor. Each p-channel MOS transistor includes a parasitic diode formed between its source and drain. The drain of the discharge control switch 4 is connected to the drain of the charge control switch 5a.
The positive terminal of the battery 1 is connected to an output terminal t1 via the control switches 4, 5a. The negative terminal of the battery 1 is connected to the ground GND and an output terminal t2.
The charge control switch 5a is controlled based on a charge control signal Cout of the control unit 3. The discharge control switch 4 is controlled based on a discharge control signal Dout of the control unit 3.
The cells 2a, 2b, 2c of the battery 1 are each connected to a cell voltage detection circuit 6 incorporated in the control unit 3. The cell voltage detection circuit 6 includes three comparators 7a, 7b, 7c. The comparator 7a detects voltage V2a between terminal BH and terminal BM. The comparator 7b detects voltage V2b between terminal BM and terminal BL. The comparator 7c detects voltage V2c between terminal BL and the GND terminal.
The output signals of the comparators 7a, 7b, 7c are each provided to positive input terminals of an overcharge detection circuit 8 and to negative input terminals of an overdischarge detection circuit 9. A charge reference voltage VTH is provided to a negative input terminal of the overcharge detection circuit 8. A discharge reference voltage VTL is provided to a positive input terminal of the overdischarge detection circuit 9.
The overdischarge detection circuit 9 provides the discharge control signal Dout to the gate of the discharge control switch 4. The overcharge detection circuit 8 provides the charge control signal Cout to the gate of the charge control switch 5a.
The control unit 3 includes a bias generation circuit 10. When the battery 1 supplies the bias generation circuit 10 with power supply voltage Vcc, the control unit 3 is activated.
When any one of the cell voltages V2a, V2b, V2c is higher than the charge reference voltage VTH, that is, in an overcharged state, the charge control signal Cout is high and the discharge control signal Dout is low. Thus, the discharge control switch 4 is activated and the charge control switch 5a is deactivated. Accordingly, charging is prohibited.
In this state, a discharge route, which includes the parasitic diode of the discharge control switch 5a, the discharge control switch 4, and the battery 1, is formed between the output terminals t1, t2. Accordingly, if a portable electronic device is connected between the output terminals t1, t2, the battery 1 provides a current to the portable electronic device. This lowers each cell voltage.
When all of the cell voltages V2a, V2b, V2c are included between the charge reference voltage VTH and the discharge reference voltage VTL, that is, in a normal state, the charge control signal Cout and the discharge control signal Dout are both low. This activates both of the control switches 4, 5a and enables charging and discharging of each cell.
When charging the battery 1, constant current charging is performed. Since the charging voltage is significantly greater than the threshold voltage of the charge control switch 5a, the ON resistance of the charge control switch 5a is small. In contrast to constant voltage charging, the current value in constant current charging is greater. However, the ON resistance of the discharge control switch 5a is smaller. Thus, the voltage between the source and drain of the switch 5a is lower. As a result, the power consumption in the charge control switch 5a decreases and the charge control switch 5a is not heated.
When any one of the cell voltages V2a, V2b, V2c is lower than the discharge reference voltage VTL, that is, in an overdischarged state, the charge control signal Cout is low and the discharge control signal Dout is high. This activates the charge control switch 5a and deactivates the discharge control switch 4. Accordingly, discharging is prohibited.
In this state, the parasitic diode of the discharge control switch 4 forms a charge route between the output terminals t1, t2. This enables charging. If a charger is then connected between the output terminals t1, t2 and charges the battery 1, which is in an overdischarged state, the cell voltages increase. This provides power to the portable electronic device.
If the battery 1 is charged when any one of the cells 2a, 2b, 2c is in an overdischarged state, the charge/discharge control circuit 50 performs constant current charging. In the prior art, when a lithium ion battery is charged, constant current charging is performed if the power supply voltage Vcc is low. When the power supply voltage Vcc becomes equal to a predetermined voltage (e.g., 12.6V), constant current charging is switched to constant voltage charging. This is because constant current charging charges the battery more quickly, since the charging current in constant current charging is greater than that in constant voltage charging.
When constant current charging is performed in an overdischarged state, such as when the level of the power supply voltage Vcc is extremely low (e.g., Vcc≈0), the charging voltage becomes low in comparison to normal constant current charging. During constant voltage charging, the charging voltage of a typical charger is set at 12.6V. However, during constant current charging, the charging voltage is controlled in accordance with the level of the power supply voltage Vcc.
Thus, when the discharge reference voltage VTL of the overdischarge detection circuit 9 is 2.5V and the cell voltages V2a, V2b, V2c are each 3V (normal state), the power supply voltage Vcc is 9V. In this state, the charging voltage is significantly greater than the threshold voltage of the charge control switch 5a, which is typically 4V.
When the level of the power supply voltage Vcc decreases to a value close to 0V (overdischarged state), the charging voltage decreases to 4V and becomes equal to the minimum voltage between the source and gate of the charge control switch 5a that enables activation of the charge control switch 5a. In this state, the ON resistance of the charge control switch 5a is large. Thus, if the voltage drop at the parasitic diode of the discharge control switch 4 is 1V, the voltage between the source and drain of the charge control switch 5a is 3V.
When the charging current is 1 A, the power consumption of the charge control switch 5a is 3 W and thus large. Hence, in the charge/discharge control circuit 50, the charge control switch 5a is heated when the battery 1 is charged in an overdischarged state, due to a large power consumption in the charge control switch 5a.